This invention relates to a PID control unit, and more particularly to an improved self control type PID control unit capable of automatically setting PID parameters to optimum values in accordance with a controlled system.
In a conventional PID control unit, PID control parameters are determined by an auto-tuning method employing a limit cycle in which a response wave form is oscillated by applying a predetermined manipulated variable to a controlled system and PID control parameters are set by identifying the characteristics of the controlled system in view of the response wave form. Alternatively, this is done by a step response method in which a step manipulated variable is applied to a controlled system, and PID control parameters are set by identifying the characteristics of the controlled system in view of its response wave form and thereafter a conventional closed loop control is executed. The conventional auto-tuning method, however, has the disadvantages that there are some controlled systems to which it cannot be applied or, and identification of the parameters consumes time since the response wave form is oscillated. The step response method needs an experimental operation prior to the common closed loop control to identify the characteristics of the controlled system.